There exists what is known as selective area growth (SAG) technique that utilises insulating film patterning masks in integrally fabricating, on the same semiconductor substrate, such semiconductor photonic devices as semiconductor laser, optical modulator, optical switch, photo detector and optical amplifier of different functions. The selective area growth technique involves primarily forming insulating film patterning masks over the semiconductor substrate so as to permit vapour phase growth of semiconductor crystals in unmasked areas, i.e. exposed areas of the substrate. During manufacture of target semiconductor photonic devices, the width of the insulating film mask and that of the exposed area over the semiconductor substrate are varied in the light transmission direction of these devices, and vapour phase growth of an alloy semiconductor is effected. This causes alloy semiconductor layers of different growth layer compositions and of different layer thickness to be formed automatically in the same process and in accordance with the width of the insulating film mask and that of the exposed area. This is because the density gradient in vapour phase of various materials that contains the atoms constituting alloy semiconductor crystals, and the effective diffusion length involved, vary from material to material.
The effective diffusion length mainly consists of two mechanisms, surface diffusion and re-diffusion. An atom, e.g. indium (In), that comes in contact with the surface of the mask, may be subject to the surface diffusion mechanism where the atom migrate along that surface until it finds a suitable substance to attach to, e.g. indium phosphide (InP). The atom may, on the other hand, be subject to the re-diffusion mechanism where the atom re-diffuses from the surface and float around until the atom collide with another atom. This collision causes the atom to drop to the surface again where it will attach if there is a suitable substance, as described earlier, or be subject to the surface diffusion or re-diffusion mechanism again. The re-diffusing mechanism is the important part in SAG.
The average distance an atom moves before it attach is called diffusion length. The diffusion length on a substrate of InP is approximately 1 .mu.m for surface diffusion and approximately 10 to 100 .mu.m for re-diffusion, dependent on pressure during SAG.
Different atoms belonging to the same group of element, e.g. group III element, may have different diffusion lengths, for instance, gallium (Ga) has a considerable longer diffusion length, approx. 110 .mu.m, compared to indium (In), approx. 15 .mu.m. These values is temperature and pressure dependent, but the ratio between them is more or less constant. The difference in diffusion length will cause a change in the composition of an epitaxially grown material, consisting of atoms belonging to the same group of element with different diffusion lengths, close to the masks. Furthermore, an increased amount of material will appear close to each mask due to diffusion from the surface of the mask.
U.S. Pat. No. 5,543,353 by Makoto et al. disclose a method for manufacturing devices, such as a laser and a modulator, in a single step using a single mask having different mask widths in the light transmission direction of these devices.
During selective area growth in a reactor, variations in the composition of a grown layer may appear due to the type of reactor used, for instance with an AIXTRON reactor equipment. A large variation will occur when the substrate is fixedly mounted in a reactor chamber and the gases, used for epitaxial growth of a waveguide layer, for instance, InGaAsP or InGaAs, are introduced in the chamber from one direction. The variations can be detected and measured by photo luminescence measuring techniques, where a variation in band-gap energy of the waveguide layer is detected and presented as a wavelength variation across the substrate. An example of this wavelength variation is shown in FIG. 1 and is described in more detail below.
When manufacturing photonic devices, such as a laser and a modulator, in different steps and in the same or different reactor, the difference in wavelength between the photonic devices may vary dependent of the position of the photonic device on the substrate. This results in a low yield of functioning devices on the substrate, since the wavelength difference, so called detuning, between the laser and the modulator is important.